The present invention relates generally to subterranean well construction, and more particularly, to improved darts and methods of using these darts in subterranean wells.
During the drilling and construction of subterranean wells, casing strings are generally introduced into the well bore. To stabilize the casing, a cement slurry is often pumped downwardly through the casing, and then upwardly into the annulus between the casing and the walls of the well bore. One concern in this process is that, prior to the introduction of the cement slurry into the casing, the casing generally contains a drilling or some other servicing fluid that may contaminate the cement slurry. To prevent this contamination, a subterranean plug, often referred to as a cementing plug or a “bottom” plug, may be placed into the casing ahead of the cement slurry as a boundary between the two. The plug may perform other functions as well, such as wiping fluid from the inner surface of the casing as it travels through the casing, which may further reduce the risk of contamination.
Similarly, after the desired quantity of cement slurry is placed into the well bore, a displacement fluid is commonly used to force the cement into the desired location. To prevent contamination of the cement slurry by the displacement fluid, a “top” cementing plug may be introduced at the interface between the cement slurry and the displacement fluid. This top plug also wipes cement slurry from the inner surfaces of the casing as the displacement fluid is pumped downwardly into the casing. Sometimes a third subterranean plug may be used, to perform functions such as preliminarily calibrating the internal volume of the casing to determine the amount of displacement fluid required, for example, or to separate a second fluid ahead of the cement slurry (e.g., where a preceding plug may separate a drilling mud from a cement spacer fluid, the third plug may be used to separate the cement spacer fluid from the cement slurry), for instance.
In certain applications, for example, when drilling offshore, the casing string may be lowered into the hole by a work string, which is typically a length of drill pipe. Because most subterranean plugs are too large to pass through the work string, sub-surface release (“SSR”) subterranean plugs are used. These plugs are often suspended at the interface of the drill pipe and the casing string, and are selectively released by a remote device when desired. Because SSR subterranean plugs are suspended at the interface between the work string and the casing, fluids must be able to pass through the plugs. However, when used to prevent contamination as described above, the channels through the plugs must be selectively sealed.
Several methods are known in the art for sealing the channels through SSR plugs. For example, if the channel is funnel-shaped, then a weighted ball may be dropped into the funnel in the plug to seal it. Another method involves a positive displacement plugging device, often referred to as a “dart.” Darts generally comprise two or more rubber “fins” that flare outwardly from a mandrel. Such fins are generally sized so as to engage the inside wall of the pipe in which they are deployed. Because its fins prevent a dart from free-falling to the plug, a pressure differential usually is applied to force the dart to the plug.
When used to release subterranean plugs, the fins of a dart must collapse or compress sufficiently to allow the dart mandrel to advance through the work string and reach the intended plug. In some instances where there is a plurality of subterranean plugs, each succeeding plug may have a successively smaller minor diameter channel such that successively larger dart noses can be used to release the subterranean plugs in sequence. Thus, a particular dart must be capable of collapsing to a small enough diameter to reach an intended plug. Several problems, however, have been encountered with conventional darts in such applications. For instance, when a conventional dart has fins that are properly sized to engage the inside wall of the work string, such fins may approach an equivalent solid mass when compressed while passing through the minor diameter of successively smaller plugs; accordingly, excessive pressure may be required to push the dart (having fins in such compressed state) to the desired plug. Using excessive pressure is undesirable, because such excessive pressure may cause the cementing plug to be released prematurely and/or out of the desired sequence. Moreover, a dart with easily-compressible fins generally does not adequately engage the inner wall of the drill string and, therefore, does not act as an effective wiping device.